I. Field of the Invention
The present invention relates to apparatus and methods used for continuous process etching of workpieces by a gas discharge plasma.
II. Description of the Prior Art
Gas plasma etching is a process applicable to etching semi-conductor devices and to etching or desmearing holes in printed circuit boards. The process provides a chemical reaction to remove selected portions of patterns formed on semiconductor device or to remove unwanted organic material from holes drilled in printed circuit boards. Such debris is referred to as "smearing" and results from the drilling process in which the insulating base material in a multilayer printed circuit board, or other foreign material, is forced by the action of drills used in forming holes in those boards to extend over the conductive inner circuits. Such smearing can prevent proper electrical connection.
To correct the problem of smearing it has become common practice for manufacturers of such multilayer printed circuit boards to apply an etching agent to the drilled board to etch the smearing material from the holes and thus provide clean connections among the circuit layers. In the past this etching procedure has been done through the use of a wet process, using liquid acids and similar materials. However, the wet processes have created serious problems of disposal of the highly toxic by-products.
To overcome certain of the disadvantages of the wet process etching various techniques have been developed for etching workpieces such as printed circuit boards or semiconductor wafers, or the like, utilizing an etchant material in the form of a gas plasma activated by the application of radio frequency energy to certain combinations of gases. These plasma etching techniques have materially reduced the quantity of toxic waste by-products to be disposed and have materially reduced the environmental danger resulting from the etching. However, most of the processes used in gas plasma etching are batch processes in which a batch of workpieces are inserted into a chamber that is subsequently evacuated and then have introduced thereinto the gas plasma for etching. Such batch processing has significant disadvantages because of the necessity to purge the chamber and completely open it for removal of processed workpiece and insertion of additional workpieces to be processed. One prior art piece of apparatus, disclosed in Yamamoto U.S. Pat. No. 4,151,034, teaches a modified batch process in which batches of semi-conductor wafers are placed into a feeding chamber ahead of the processing chamber and fed sequentially from that feeding chamber into the processing chamber, out of the processing chamber into a rebatching receiving chamber that collects wafers until a batch is completed, which batch is then removed. This apparatus of Yamamoto et al is directed toward semiconductor wafer workpieces and suffers several disadvantages due to the large feeding and receiving chambers necessary to collect complete batches of workpieces.